1. Background of the Invention
This invention relates to the use of hydrogen peroxide reacted with strong reducing agents or stronger oxidizing agents to produce hydroxyl radicals. More specifically, the invention relates to utilizing in-situ generation of hydroxyl radicals to effectively oxidize odor and/or noxious compounds found in the food processing industry, such as poultry, beef, or fish processing, as well as rendering of meat, fish, or fowl by-products. This invention also relates to the use of hydroxyl radicals in the reduction of volatile organic compounds in aqueous gas scrubbers.
2. Description of Related Art
In the processing of poultry, beef, and fish, the large volume of organic material processed, as well as secondary processing (rendering), can generate large quantities of odiferous gases including organic sulfides, thiols, amines, alcohols, inorganic sulfides, ammonia, and simple carboxylic acids. These compounds are usually the result of biological action on the organic materials being processed. The odors produced are offensive and can travel significant distances to surrounding real estate. In other industries, such as chemical processing, paint production, wastewater treatment, etc., noxious compounds, such as volatile organic compounds (VOCs), are produced and are subject to environmental air quality regulations.
These gases are usually captured by a water media in air scrubber systems. In an air scrubber system, typically, air from the processing step is evacuated into a tower where water, broken into droplets either by contact with mixed media or distribution channels, absorbs the odiferous and noxious gas compounds. This water is recirculated and discharged typically to wastewater treatment systems. U.S. Pat. No. 6,015,536 to Lokkesmoe et al., provides a detailed description of the available air scrubbing systems on the market and is hereby incorporated by reference herein.
It can readily be seen that the scrubbing water media will quickly saturate with the offensive gases and lose its absorbing potential. At this point, the water has an intense disagreeable odor. Additives are commonly injected into the scrubbing water stream to reduce the odor content of the aqueous scrubbing media. The water is either dumped to the wastewater treatment facility or a portion is withdrawn to the wastewater facility and fresh makeup water is added to account for the difference.
It can also be seen that as the water saturates with gases, particularly nitrogen-bearing gases (ammonia), the pH of the water will rise proportionally. This marked increase in pH reduces the solubility of the gases causing them to flash to the atmosphere. This results in a decrease in the efficiency of the gas transfer to the water media.
Numerous attempts have been made to reduce the odor components in the air scrubbing system. Some technologies attempt to reduce the odor by injecting a maskant, which is a stronger, more pleasing odor compound. These maskants are extremely expensive, and the duration of the effectiveness is very short. These compounds do nothing to the actual structure of the odor molecule.
Maskants are also “fogged” or injected into spray orifices under high pressure to create a small droplet mist effect. These misted materials are directed into the atmosphere around the odor causing process. The misted materials either mask the odor or combine with the odor-causing molecule in the atmosphere to temporarily lower the offensive odor. These compounds, which are usually essential oils, are then blown with the prevailing winds. These materials are limited by extreme cost and do nothing to actually affect the odor-causing molecule.
U.S. Pat. Nos. 4,443,342 and 4,595,577, to Stas et al., describe a treatment method using hydrogen peroxide and copper sulfate as the catalyst for treatment of wastewater and gases containing organic sulfur compounds in a pH range below 6.5. It is well known to those of skill in the art that the efficiency of hydrogen peroxide as an oxidizer is increased in the pH range of 3 to 6.5. Stas et al. also describe using hydrogen peroxide in acidic aqueous media with 1 to 5 ppm of ferric sulfate as a catalyst when comparing the efficacy of the use of copper. This catalyst choice, in the amount used, only increases the efficiency of the hydrogen peroxide and does not effectively reduce the hydrogen peroxide to free radicals in the quantities that would be needed to oxidize odor components to soluble compounds to enable their removal.
Hydrogen peroxide by itself has only moderate success in air scrubbing systems. It reacts very slowly and is limited in the number of organic molecules it can oxidize. Some odor reduction can be achieved using hydrogen peroxide, but it is usually via microbiological control or an increase in oxygen content of the aqueous scrubbing media.
Halogen donors such as chlorine dioxide, chlorine gas, sodium hypochlorite, and hypobromous acids have had limited success in the art. The low electronegativity of these halogens (1.0 to 1.7 volts) limits their ability to oxidize odor constituents down to simple soluble compounds. Therefore their odor removal efficiency is low compared to the quantity needed. The use of halogen donors is falling under environmental scrutiny due to the formation of haloamines as well as trihalomethanes. Use of these halogens in aqueous streams eventually contributes to trihalomethanes and haloamines in surface waters.
U.S. Pat. No. 6,015,536, to Lokkesmoe et al, describes the use of peroxyacid compounds at a pH of 3 to 6 for odor reduction in air scrubbers. Peroxyacid, namely peracetic acid, is used as an oxidizer of odor causing molecules, and a reduction of odors from 5 to 50% is described. However, the large doses of peracetic acid needed preclude higher odor removal rates due to cost as well as the contribution of a pungent odor from the acetic and peracetic acids. The peracid compounds also lack sufficient electronegative potential to break up odor causing compounds to the degree needed for greater than 50% removal rates.
Ozone has been used with limited success in aqueous gas scrubbers. Ozone has an inherent high capital expenditure cost and is difficult to utilize the ozone gas in aqueous media. It is difficult to force enough gas into contact with the aqueous media to effectively oxidize odor compounds into simple soluble reduced-odor compounds. Ozone is also characterized by large electric utility costs associated with corona discharge type ozone production units.
Inorganic percompounds, such as percarbonates, persulfates, perborates, and permangenates, have demonstrated odor control potential. These compounds, however, are notoriously slow to liberate oxygen in cold water at elevated pH.
There exists a need in the art for a treatment process that has a high enough electronegative potential to reduce substantially all odor and/or noxious compounds to simple, soluble, reduced-odor/noxious, or odor/noxious-free compounds. This treatment process would offer even greater advance in the art if the process could also eliminate or greatly reduce the high cost of treating the scrubber water effluent in the wastewater treatment process.